Magnesium in its ionic form is essential to many physiological processes. It is one of the most abundant cations in the body and, next to potassium, it is the most prevalent intracellular ion. It plays a vital role in carbohydrate and lipid metabolism by serving as an activator of adenosine triphosphate (ATP) in the transfer of energy rich phosphate. It is also essential as an activating ion for many enzymes involved in lipid, carbohydrate and protein metabolism. In muscle tissue, magnesium has a significant influence on neuromuscular apparatus.
The amount of magnesium in the body is particularly significant. Decreased levels of magnesium in the body produce muscle irritability which, if not corrected, can result in involuntary muscle spasms and convulsions. On the other hand, increased levels of magnesium can result in a loss of deep tendon reflexes, a loss of touch, temperature and pain sensation, respiratory failure and cardiac arrest.
Therefore, it has been long recognized that for suitable diagnosis and treatment of various ailments, the accurate and rapid measurement of magnesium ions is important. In addition, it is also important in many environmental monitoring programs and manufacturing processes that magnesium be accurately measured.
Colorimetric methods are known for the determination of the concentration of magnesium ions in various fluids, e.g. groundwater, seawater, wastewater, manufacturing liquids and biological fluids. These methods usually involve adding a reagent to the fluid which forms a colored complex with any magnesium ions present. The complex absorbs electromagnetic radiation at a characteristic wavelength different from that of uncomplexed reagent.
The known methods for determining magnesium have various drawbacks. The fluids to be tested often contain various materials which interfere with the complexation of magnesium ions with a complexing dye. For example, proteins and calcium ions present in fluids can also complex with the dye thereby causing an interference.
Hydroxy-substituted cyanoformazan derivatives have been used in the analysis of metal ions in fluids for some time, as described by Budesinsky et al, Inorg. Chem., 10(2), 313-317 (1971) and Podchainova et al, Zhur. Analiticheskoi Khimii, 32(4), 822-832 (1977). These references describe the complexation properties of several cyanoformazans with various metal ions. In the Chinese journal Chemical Reagents, 4(4), pp. 219-222 (1982), the effect of surfactants on the water-dispersibility of 1,5-bis(2-hydroxy-5-sulfophenyl)-3-cyanoformazan and 1,5-bis(2-hydroxy-5-chlorophenyl)-3-cyanoformazan was evaluated. However, use of these compounds presents problems. Both must be used at relatively high pH, i.e. greater than 11, for greatest sensitivity for magnesium ions. Otherwise, their selectivity for magnesium is low. Further, the 5-sulfophenyl derivative exhibits high background in an assay for magnesium ions.
A recent advance in clinical chemistry was the development of analytical elements useful in dry assays. Examples of such elements are described, for example, in U.S. Pat. Nos. 3,992,158 (issued Nov. 16, 1976 to Przybylowicz et al) and 4,258,001 (issued Mar. 24, 1981 to Pierce et al).
Early attempts to assay liquids for magnesium ions with dry analytical elements were unsuccessful. Known dyes which complex magnesium ions also complex with calcium ions which are often present in liquids to be tested. Further, it was found that proteins adversely affect the assay by complexing with the dyes and biasing the results. Yet it would be desirable to have a dry assay which avoids the problems encountered with both known solution and dry assays.